I don't think Barcelona will be a big threat to Real Madrid in tonight's match because both clubs are not in the same category in terms of players' motivation and other aspects such as you mentioned some key players from the Barcelona team are on injuries all these factors will contribute to making the match tough for Barcelona and Real Madrid have a highly motivated team.
Real Madrid must be wary of the strength of Xavi's team, even though there are no players capable of playing creativity in the midfield, but the role of defenders and defensive midfielders is needed to break possession and make Barcelona's midfield unable to play optimally. Luca Modric and Toni Kroos can play this role, apart from being very experienced, these two players also really understand Barcelona's attack patterns.
Lewandowski is out with an hamstring injury, while Pedri is out with a thigh injury, so the both of them are going to miss the El clasico tomorrow, Xavi would like to play with them than without them, but i believe they have enough quality players and they can replace them. The game is going to be difficult, but it would not be as difficult as it would have been if it was an El Clasico in the La Liga. The Copa del rey is important, but not as important as the La Liga, though progressing in the Copa del rey would help the winner to grow their confidence in the La Liga. Even if Barca have some missing players, i still feel they would win tomorrow.
Barcelona has confirmed the squad that will be brought to face Real Madrid in the El Clasiko match, there is no Lewandowski, Pedri and Dembele will make Barcelona's strength a little weaker, Real Madrid cannot underestimate the strength of Barcelona which is not strengthened by them. The midfielders available for Barcelona at this time were also quite problematic in several previous matches, so there was no creativity that could be provided by them and makes it difficult for attackers to score goals.
If there is no creativity in the midfield, it will be very difficult for Barcelona to score goals in the El Clasico match, especially since their goal machine Lewandowski cannot appear in this match and I doubt very much that Barcelona will perform optimally in this match.